German cuirassier officer’s armor with some bullet dings, circa 1620-1630
from Hermann Historica
‘Proofing’ armor by shooting them is well attested from the mid-14th
century onward, and particularly in the 15th and 16th century. The armor would be shot by a projectile weapon at a certain range, and
would pass the test if it was not penetrated. This armor was thus
‘proofed’ and was said to be ‘armor of proof’. Originally the testing
used strong crossbows, and shifted to using guns in the 16th century.
Thus earlier armor do no have visible bullet marks but these become
more common into the 16th century.